1. Field of the Invention
The present invention relates a developing device that forms a toner image on a predetermined image carrying body and to an image forming apparatus provided with the same.
2. Description of the Related Art
Image forming apparatuses that use an electrophotographic method, such as copiers, printers, fax machines, and multifunction machines that combine these, include a developing device supplying a toner to an image carrying body (for example, a photosensitive drum or transfer belt) to form a toner image on the image carrying body.
The developing device includes, as fundamental elements, a developer storing portion which stores a developer constituted by a toner and a carrier, a developing roller which forms a toner image on an image carrying body by receiving the developer from the developer storing portion and supplying the developer to the image carrying body, and a regulating blade which is arranged in opposition to a circumferential surface of the developing roller and regulates a layer thickness of the developer on the developing roller.
In the thus configured developing device, a gap of a predetermined dimension is set between a leading edge surface of the regulating blade facing the developing roller and the circumferential surface of the developing roller, and a developer layer of a uniform layer thickness is formed on the circumferential surface of the developing roller by causing the developer that has adhered to the circumferential surface of the developing roller from the developer storing portion to pass through this gap.
However, the developer tends to accumulate in a space on an upstream side from the regulating blade as viewed from the rotation direction of the developing roller, and within this space, toner deterioration due to bury of external additives to the toner or separation of external additives from the toner, or carrier deterioration due to adhesion of toner external additives to the carrier occurs since, for example, the developers rub against each other. When the developer deteriorates in this manner, the toner cannot be favorably charged. Furthermore, when the developer, which is a magnetic substance, accumulates on the upstream side of the regulating blade, it is difficult to focus a magnetic line, which is produced between the regulating blade and the developing roller, on the regulating blade, and the regulating force of the regulating blade does not stabilize. As a result, it becomes difficult to form a uniform layer thickness of developer on the circumferential surface of the developing roller, and thus also difficult to form a favorable toner image.
A first prior art and a second prior art are known as techniques that inhibit the above-mentioned deterioration of the developer. In a developing device of the first prior art, a carrier returning member is arranged on an upstream side from the regulating blade as viewed from the rotation direction of the developing roller and at a position adjacent to the regulating blade. The carrier returning member is provided with a guiding conveyance surface that opposes the circumferential surface of the developing roller, and the guiding conveyance surface is tilted so that a space between the guiding conveyance surface and the developing roller gradually becomes larger from the regulating blade toward the rotation direction upstream side.
The developer that adheres to the circumferential surface of the developing roller from the developer storing portion advances into this space due to rotation of the developing roller, and is thus gradually pushed back to the developer storing portion by the tilted surface of the guiding conveyance surface. Due to this, a large amount of developer is suppressed from accumulating on the upstream side from the regulating blade in the rotation direction of the developing roller.
In a developing device of the second prior art, an elastic member is arranged in the upstream side space where the developer tends to accumulate. The elastic member is arranged so as to occupy the upstream side space in a state in which the elastic member is in close contact to the upstream surface of the regulating blade as viewed from the rotation direction of the developing roller. Since the upstream side space is occupied by the elastic member, the developer is suppressed from accumulating there.
Furthermore, the elastic member has a conveyance amount regulating surface that opposes the circumferential surface of the developing roller. The conveyance amount regulating surface is set so as to gradually be more apart from the developing roller toward the upstream side in the rotation direction of the developing roller, and therefore the developer that adheres to the circumferential surface of the developing roller from the developer storing portion advances into the upstream side space due to rotation of the developing roller, and is thus gradually pushed back to the developer storing portion by the conveyance amount regulating surface. Due to this, a large amount of the developer is suppressed from accumulating on the upstream side from the regulating blade in the rotation direction of the developing roller.
However, in the developing device of the first prior art, a step is formed between the end surface of the regulating blade and the guiding conveyance surface of the carrier returning member due to factors such as dimensional precision and installation precision of the carrier returning member, and therefore the developer tends to accumulate in the step. As described above, when the developer accumulates, the developer deteriorates and it becomes difficult to form a developer layer having a uniform layer thickness on the circumferential surface of the developing roller.
Furthermore, since the elastic member in the second prior art is a member having a thickness enough to occupy the upstream side space, in the case where the elastic member deforms due to heat produced in the developing device, the extent of thermal deformation tends to increase. When the extent of thermal deformation of the elastic member increases, a gap is produced between the elastic member and the upstream surface of the regulating blade, and the developer enters the gap and accumulates in this gap. Furthermore, when the elastic member deforms, the position of the conveyance amount regulating surface changes, and therefore in consideration of this change, the step between the regulating surface of the regulating blade and the conveyance amount regulating surface becomes undesirably larger, so that the developer tends to accumulate. Thus, as described above, when the developer accumulates, developer deterioration occurs and, as a result, it becomes difficult to form a toner layer having a uniform layer thickness on the circumferential surface of the developing roller.